1. Field of the Invention
The present general inventive concept relates to a voltage adjusting circuit, and more particularly, to a voltage adjusting circuit to generate a constant power regardless of an external environment such as a distance between a card reader and a contactless card and a contactless card and a contactless card system which include the same.
2. Description of the Related Art
Contactless card systems, and more particularly, smart card systems use contactless information recognition technology in which a card reader recognizes a smart card, which is several centimeters away from the card reader, and transmits and receives information to and from the smart card using a radio frequency.
A contactless card is supplied with power by inducing voltage from electromagnetic waves emitted by a card reader. Accordingly, the power induced in the contactless card changes depending on a distance between the contactless card and the card reader or the like. When power exceeding what is needed to drive the contactless card is induced in the contactless card, an internal logic circuit in the contactless card may malfunction or break down due to the excessive power.
To overcome this problem, constant power needs to be supplied to the internal logic circuit of the contactless card regardless of external environments.